ABOUT THIS EPISODE

David Reich, professor of genetics at Harvard, talks to Clive Cookson, the FT's science editor, about how the genomic revolution is affecting paleontology and the study of human pre-history.

English
United States

TRANSCRIPT

00:00:05Hello and welcome back to tectonic a podcast that looks at the way technology is changing our lives I'm john thornhill innovation editor at the financial times in london last week we heard from a psychologist about the fallibility of human memory and about the software tools she has designed
00:00:21to report workplace discrimination and harassment This week we look at the emerging field of ancient dna on what it tells us about our ancestors There's been past scientific revolutions in archeology The most important and the first one was the radiocarbon dating revolution which provided direct dates on ancient
00:00:40cultures and really changed the way people saw the past This is a more important revolution even than that That was david right professor of genetics at harvard and author of a book entitled who we are and how we got here he came into the ft studio to talk
00:00:56to clive cookson our science editor But how the genomic revolution is affecting paleontology on the study of human prehistory Welcome david thank you it's great to be here so before we get on to the fascinating findings of ancient dna research about prehistoric movements of human populations let's start
00:01:20with the technology that you're using and as a former organic chemistry student i'm still amazed by the survival power of what i would have thought a long time ago as rather a fragile molecule So tell us how you get dna id of old bones and other human remains
00:01:39and then how you analyze and read it the technology for getting dna out of old bones has been developing over the last thirty years led by laboratories in europe really but there's been a real breakthrough in the last ten years where it's been possible to extract whole genomes
00:01:56of similar quality to the ones we obtained from modern people from ancient specimens sometimes tens of thousands or even more than a hundred thousand years old and what we do when we get ancient dna from an old bone or an old tooth is we grind up some powder
00:02:11from the tooth in a very clean environment we have a clean room such as is used in a microchip fabrication facility that protects the sample from the people handling it We wear space suits to protect the sample from our dna We are dna bombs full of dna and
00:02:25there's so little dna in the sample that is easy to contaminate We bleach down the surfaces and the chemicals we use and we use uv light and all of these things are to protect the sample from our dna we take this powder that we've obtained we dissolve it
00:02:41in a watery solution that releases the dna we then convert the dna into a sequence a ble form and then sequence it in one of these modern sequencing machines that were developed in the late to thousands and i've made sequencing literally one hundred thousand times less expensive than
00:02:56they were before that time so what's the oldest dna that you anyone else's successfully extracted from human remains or indeed from other plant or animal remains the oldest mom alien dna is about seven hundred thousand years old it's a horse found in permafrost in alaska and the whole
00:03:16genome was obtained from it by a colleague and denmark and the oldest human much poor equality but the oldest human dna is about four hundred fifty thousand years old it's a pre neanderthal from spain on dh just to be clear to get the best results you compare the
00:03:35dna the genomes of these ancient people with those living today because you need a framework tonto of current human populations to make the most of it is that right That's usually what we do and especially when we're studying dna from ancient humans that are relatively close in time
00:03:52to the present we study present day variation and we see how these ancient people relate to the present day ones but sometimes when we're looking at sufficiently old individuals such as from ice age europe or other very old specimens we just compare the ancient samples to each other
00:04:06because they're so remote in time that present day variation is not very information rich about how they relate to each other okay well let's start discussing the results as far back in time as we can so what has ancient dna research told us about the evolution of modern
00:04:24humans both in our ancestral african homeland and then across europe and asia What have you learned The archaeological and fossil record shows that there were many groups of archaic humans as little as fifty thousand years ago in different parts of the world there's only one group of humans
00:04:42now which is modern homo sapiens sapiens but at that time there were many groups and dna has successfully been extracted from some of those archaic groups living outside of africa neanderthals and completely unanticipated group called denise evans that my colleagues in germany discovered and that i was involved
00:04:58in analyzing and so with the dna extracted from these neanderthals in europe in the denise evans in central asia it's been found that these populations we now know how they're related to us they're separated by about a half million years since they share a common ancestor we know
00:05:16that they're related to each other but very ancient lee and we know that both of them inter bread with the ancestors of people living today in different ways at least three times diddle this happen in eurasia or do you think there was similar mixing in africa it's surely
00:05:31the case that this also happened in africa africa was a place dense with different human populations it is our homeland of our particular branch of humans ancient dna has really been much more successful to date although this is beginning to change outside of africa than inside africa and
00:05:48because we don't have ancient dna it's harder to fish out where the bits of ancient archaic ancestry and modern humans are in africans but even that is possible and there's pretty strong hints that there's archaic ancestry also mixed into african populations and their descendants who lived outside of
00:06:03africa so when do you think are homos appian ancestors first moved out of africa i know the dates being put back and there was some archaeological discoveries in israel pushing it back to maybe hundred seventeen thousand two hundred thousand years ago what's your current view based on your
00:06:22work so humans first go out of africa almost two million years ago but these are very archaic humans homo erectus spreading out almost two million years ago the ancestry of non africans today is from an expansion that happened very dramatically all around the world around fifty thousand years
00:06:39and afterward but in the near east middle east there's clear evidence of modern humans like us one hundred thousand hundred thirty thousand and now even older as you said in the near east in some sense you should think of the near east as an extension of africa ecologically
00:06:53and so really the question for me often is when did the expansion out of africa and the near east happened and there is some really interesting evidence that there may have been some quite early expansion of modern humans beyond the near east and mixed into the neanderthals because
00:07:08there's bits of dna in the andr thaws that looked like they might have come from modern humans a few hundred thousand years ago so what does this say for the old out of africa scenario it's largely the correct view there was a view in the seventies that perhaps
00:07:24humans had evolved in each place in the world's in africa in the east asia and europe from the local humans who had been there for a couple of millions of years but there was a consensus from genetics and archaeology that developed in the late eighties that almost all
00:07:38ancestry of humans today comes out of africa sometime around fifty thousand years or so ago that's still largely true but all non africans have about two percent of their dna from these archaic neanderthals and some non africans have an additional few percent of dna from denise evans and
00:07:54so what's true now and what we see now is that it's a mostly out of africa scenario but there's some lineages and on africans today that are from these archaic humans who were established there for many many hundreds or thousands or perhaps even more than a million years
00:08:07before and you put in your book the hypothesis that could have been a returned to africa as part of this moving and melting process yeah i think there's been a pendulum swing toward we've been in africa our lineage the whole time effort for many many millions of years
00:08:23african great apes chimpanzees and gorillas are our closest relatives and separated from us five to ten million years ago the australopithecus scenes thie upright walking apes clearly first evolved in africa from the fossil record and homo erectus moved out of africa sometime after two million years ago and
00:08:39because there's so much africa because africa is so central to a story and because modern humans moved out of africa again sometime after fifty thousand years ago it's tempting to think that we've been in africa the whole time but i think that pendulum swing is perhaps a bit
00:08:52too far and eurasia is a rich and complex environment and could have housed our lineage for some of that time let's move on tio forty fifty thousand years ago in this great sweep of homicide peons modern humans across europe and asia wiping out the existing other populations do
00:09:13we know whether they were actually killed off by us that any sevens neanderthals and others or was it climatic factors what did them in and left us is the supreme and only species i think that's an extraordinarily important question and it's one that genetics provides some information about
00:09:31but doesn't solve genetics really can't figure out how it was that the neanderthals disappeared they certainly didn't completely disappear as they were mixed in with modern humans but we know they disappeared fairly rapidly from the archaeological record and we know they contributed only a very little bit toe
00:09:45humans living today same with denise evans i think that one of the things that's useful to think about here is that there are many many examples that we now know from ancient dna of extinct populations groups that once lived in places and no longer exist anymore and that's
00:10:00not just limited to neanderthals and denise events but also to modern humans so if we look a at the first humans we have from europe we have for example dna from a forty thousand year old individual from romania and that man was part of a population that didn't
00:10:13contribute anything to later europeans no more closely related two europeans than he is to the stations so that individual went extinct just like the neanderthals did okay well let's fast forward to much more recent times one of the most dramatic movements of population in your book is the
00:10:31great sweep of people east out of the russian steppes about four and a half thousand years ago The young naya culture moving westward right across europe and into british isles supplanting the existing population what can you tell us about that Because it's a really quite a dramatic and
00:10:49surprising discovery yes so the dna as i understand it from the archeology are an extraordinary population from an archeological point of view prior to that they lived north of the black and caspian seas and prior to the omnia thie groups who lived before them were settled in villages
00:11:07on the riverbanks and once the young knight culture arose those settlements disappeared and the only remains from the omnia largely are these large graves and the thought is that these people were living in the newly invented carts and using forces that had been newly domesticated tto go out
00:11:25into the open steppe in economically exploit the grasslands that had not been exploited before using horse and whale technology these people were very wealthy compared to the people who've lived before they were economically exploiting the landscape much better and they expanded in an extraordinary way supplanting many of
00:11:40the groups that lived before kinetically if we look at data from central europe for example germany beginning eight thousand years ago and moving to four thousand years ago we see that the first farmers beginning around seventy five hundred years ago were pretty continuous in that region until about
00:11:56forty five hundred years ago when that population is replaced about seventy percent at least by people moving in from the east derived from these the omni people a few hundred years later that happens in britain too and the replacement is even more dramatic the replacement is ninety percent
00:12:11from a population who's largely derived from these young people so there's a very great expansion of this group and it's a mystery why this expansion occurred and why they were able to replace these densely settled sophisticated farming populations who were already established for example the people who built
00:12:29stone hand here were largely replaced by this westward movement when you say replace do you think it was a ngara s ihe ve warlike invasion is that the way to look at it I think that that's probably not the right way to look at it these people were
00:12:43not organized in large scale societies there were out armies there's no evidence of such things but it may not have been particularly friendly an example of this we have unpublished data that we're working on in iberia where the contrast is even stronger in britain that population is ninety
00:12:58percent replaced but in iberia it's only twenty percent replaced by these new types of people but the y chromosomes which are inherited from father to son to son are almost completely replaced and so what you're seeing is evidence of social inequality writ in the genes where for example
00:13:14males from these incoming population havin preferential access to local females over the males that are the offspring of the local people and that happens again and again generation after generation we see this more recently in the americas where for example latinos who are mixtures of native american and
00:13:30european ancestry are also affected by this process where european males coming from europe had preferential access toe local females over the local people again and again over many generations and in african americans also in african americans about twenty percent on average of african american ancestries from europeans but
00:13:50four to one ratio of that is coming from the male side and that is well we can imagine why that is that men of the household the white slave owners having access to a female african slaves these patterns in the genes are evidence of past suffering and inequality
00:14:08and events that were quite profound when they occurred going back to britain on dth isa planting of thie people who build stonehenge and all the other megaliths it's tempting from what you've said in your book and other evidence to think of that as a rather idyllic peaceful culture
00:14:27and the people who came in from the east is being mohr aggressive spreading inequality and being generally less in twenty first liberal terms desirable is that a fair picture The first person who became super interested than the amnesia and the possibility that they spread east or that they
00:14:44had a cultural impact east was someone named maria gone but us who had a very dramatic picture of the amnesia as a very male oriented hierarchical society that was not a very nice society replacing a very female centered society that she documented in what she called old europe
00:15:00where there's a lot of female iconography and archaeological remains and venus figurines being replaced by these people and as a geneticist it's not my place to really think about these issues but we can show that the westward movement of these people had a really dramatic effect against infact
00:15:17some sort of clash or antagonism between geneticists and ancient dna practitioners such as yourself and the more traditional archaeologists and paleontologists that's a great question and i actually think it's not a clash but rather a meeting of two worlds i think archaeologists are fundamentally scientists they're deeply deeply
00:15:40interested in the past and prehistory a time when there's no writing and archaeologists are so thirsty for information about this time there's been past scientific revolutions in archaeology the most important and the first one was the radiocarbon dating revolution which provided direct dates on ancient cultures and really
00:15:56changed the way people saw the past this is a more important revolution even than that but archaeologists have embraced each past scientific revolution the practitioners of this revolution jin are not trained in archaeology so we perhaps don't always speak with the nuance that the archaeologists have in their
00:16:12own fields And so we need it's incumbent on us as geneticist to try to talk to as many archaeologists as we write our papers and to learn from our archaeologist colleague so that we can express our findings in a way that does not great against the years of
00:16:25archaeologist and allows them to think about our results in ways that make sense for their work in the case of britain for example i think it's very important to emphasize that this is not necessarily an invasion in fact it's unlikely to be an invasion with a large army
00:16:40coming in and that somehow we have to think about what kinds of human processes could have in such a short time just a couple of hundred years replaced ninety percent of the population now your book tells us in rich and marvelous detail what ancient dna tells us about
00:16:57the movement of populations but you say much much less about what it tells us about the physical characteristics of these people moving around the world what they looked like how they might have thought what they might have spoken they talk a bit about linguistics do you stay away
00:17:13from that Because it's all too sensitive or just because the genes are not yet telling you enough about the physical on biological characteristics of these people i think it's very much the ladder i don't have a problem with talking about sensitive things and i think it's important to
00:17:29actually discuss sensitive things but in fact genetics is very much at the beginning in terms of being able to make predictions about how people looked in people's health if you scan your genome even a modern person and try to predict your risk for heart disease or diabetes or
00:17:43cancer you get a very poor prediction how much worse it is for ancient peoples we really are very poorly able to predict how ancient people looked from genetic data so well you can say some trivial things like a prediction about the hue of a person's skin or the
00:17:57color of their eyes we can say almost nothing about how they thought or behaved and so really genetics is only at the very beginning and being able to tell stories those things although that will change over time yet your book and ancient dna research more generally is to
00:18:14some extent rekindling a debate about the modern scientific basis of race does race have any scientific meaning it all in twenty eighteen do you think i think race doesn't have any scientific meeting at all Race is a social concept it emerges out of categories that change over time
00:18:34people who would be classified as being of one race black for example in brazil would not be classified as being black in the united states for example and vice versa and even within the united states it's changed over time but when people are categorized into quote races today
00:18:49some of those categories do correlate strongly toe really genetic average differences amongst populations and so because there are real genetic average differences amongst populations in the world than human populations have been separated for substantial periods of time There is the opportunity for there to be substantial average genetic
00:19:07differences across human populations with regard to a range of traits and so we need to think about that and what's your thinking about it telling you so far Well i think we know almost nothing as i mentioned before about what the actual genetic basis for the stereotyped differences
00:19:24amongst human populations and if we know anything it's almost certainly that those stereotypes don't have a genetic basis that they're almost call culturally determined and quite possible The genetics is pushing the direction opposite to the stereotype However people of west african ancestry in european ancestry for example are
00:19:39from populations that have been separated for seventy thousand years or more and that's more than enough time for natural selection in evolution tto change average traits across populations so there is opportunity for differences to arise Humans are not all genetically identical and human populations are not all genetically
00:19:56identical And so i think that it's not appropriate to say that the differences between human populations are so trivial that there can't be any differences that are arise i think that it's important teo prepare our society tohave robust enough way of talking to be able to handle any
00:20:12differences that are discovered and they will be discovered over the coming years even though we don't yet know what they are well thank you very much david it's being a huge pleasure reading your book and i'm talking to you about some of your findings and good luck with
00:20:25your future research Thank you it's Been such a pleasure talking with you We'll be back next week with another episode of tectonic In the meantime if you'd like to comment on today's show or suggest any topics you'd like us to cover in future episodes please email us a
00:20:39tectonic at ft dot com Don't forget to subscribe to our show on your favorite podcast app and if you write a review that will help other people find us too Thanks for listening This episode of tectonic was produced by fiona simon Support for this financial times podcast comes
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